1. Field of the Invention
This invention relates to a fluidized catalytic cracking process and apparatus for resid in general and heat integration of reaction-regeneration sections of the resid FCC unit in particular.
2. Description of the Related Art
Fluidized catalytic cracking (FCC) is one of the most important conversion processes in the refining industry. FCC was initially designed for a silica-alumina matrix type catalyst with a dense bed reactor-regenerator system. However, since the introduction of zeolite type catalysts, the FCC reactor has been converted to all riser cracking with significant reduction in riser residence time and catalyst inventory.
With further improvement in the catalyst composition, FCC could be run at higher metal level (5000-7000 ppm nickel and vanadium) on an equilibrium catalyst. Simultaneously, for reduction of bottom of the barrel, conventional FCC units were modified for handling heavy residue, e.g., atmospheric and vacuum resid, etc. The modification involved the improvement in feed atomization, quick riser termination and quench, better catalyst striping, two stage catalyst regeneration, external catalyst cooler, catalyst and air distributors, etc.
In a resid FCC unit, the feed is preheated to 150-250.degree. C. and injected radially at the bottom of the riser with steam as a dispersing medium. The contact time of the riser is kept in the range of 2-6 secs and the temperature in the riser bottom and top normally remains around 540-580.degree. C., respectively. Suitable riser terminator devices are attached at the end of the riser to quickly disengage the catalyst from the product vapor. The catalyst is guided to a bubbling bed stripper where steam at the rate of 2-5 kg/1000 kg of catalyst is injected at the bottom of the stripper to remove the entrapped hydrocarbon vapor from the catalyst. The product vapor after the riser terminator is quenched or guided to the second stage cyclone and finally to the main column fractionator. The stripper catalyst is fed to the 1st stage of regenerator which works in the temperature range of 650-690.degree. C. The carbon on catalyst is significantly reduced (70-80%) in this stage which then is pneumatically conveyed to the second stage regenerator where the temperature is kept much higher (710-740.degree. C.) with sufficiently excess oxygen for near complete removal of carbon (&lt;0.05%) on catalyst. The regenerator catalyst from the second stage of the regenerator is fed to the riser bottom through regenerated catalyst slide valve where the catalyst circulation rate is controlled to maintain the riser top temperature. Typically, the resid FCC unit operates at a 5-8 cat/oil ratio. In some resid FCC units where the quality of resid (Conradson cokes 3-4%), the catalyst in the regenerator is cooled in an external catalyst cooler to maintain over all heat balance of the unit.
Although many modifications in the original FCC unit have been made earlier to process residues, such resid FCC units can not handle very heavy residues where the Conradson carbon is more than 30-50 ppm. Several problems are associated in the known resid FCC units to economically process resid. These problems are as follows:
i) Excessive coke with the resid produces a large amount of excess heat and therefore the heat balance of the reactor regenerator is disturbed.
ii) Higher metal level on the resid leads to significant deactivation of the catalyst and requires a very large catalyst addition rate to keep the metal level or equilibrium catalyst in an acceptable range.
iii) Crackability of some of the residue, in particular, aromatic residue, are not quire good. Sufficient residence time for such residues are required in the riser and the extra coke generated from such aromatic residue cracking is required to be handled.
iv) Poor strippability of the catalyst: Strippability of the heavier unconverted residue inside the catalyst pores is not al all efficient.
v) SO.sub.2 emissions from present resid FCC units are very high and present resid FCC conditions are not very conducive for efficient functioning of SO.sub.x removal additives.
vi) NO.sub.x generation in the present resid FCC unit is quite high due to high temperature regeneration.
These problems are further discussed in the following sections.
Excess Coke Formation Associated Higher Regeneration Temperature
Coke make in FCC is the most critical parameter to maintain the heat balance. Coke produced in the riser is burnt in the presence of air in the regenerator. The heat produced through exothermic coke burning reactions supplies the heat demand of the reactor, i.e., heat of vaporization, and associated sensible heat of the feedstock, endothermic heat of cracking etc. Typically, the coke yield in a conventional FCC unit with vacuum Gas Oil remains in the range of 4.5-5.5 wt %. The heat produced from burning (complete combustion) is sufficient to supply the reactor heat load. However, in a resid FCC unit, since the feedstock contains large amounts of coke precursors with higher amounts of Conradson coke and aromatic rings, the coke make is significantly increased which in turn increases regenerator temperature from 650-860.degree. C. in conventional FCCUs to 720-250.degree. C. in residue crackers.
The higher regenerator temperature has multiple deleterious effects in resids FCC units. However, the following are the major issues involved in high temperature regeneration:
i) High regenerator temperature reduces the catalyst circulation rate for a given riser top temperature to maintain in the reactor heat balance. Thus, the effective cat/oil ratio drops significantly resulting in reduced conversion.
ii) Higher regenerator temperature significantly increases catalyst deactivation both due to the metal, as well as hydrothermal factors. In fact, a regenerator temperature beyond 700.degree. C. exponentially increases the zeolite crystallinity loss which is further aggravated in the presence of vanadium impurities on the catalyst. The maximum vanadium level which can be tolerated in FCC depends on the regenerator temperature. The tolerable vanadium level can be significantly improved by 4-5 times if regenerator temperature is reduced from say 730.degree. C. to 680.degree. C. Similarly, hydrothermal deactivation of catalyst also drops significantly with regenerator temperature reduction.
iii) High regenerator temperature is not conducive for SO.sub.x additive which works better at moderate regenerator temperatures (680-700.degree. C.). Similarly, NO.sub.x emission is significantly increased beyond regenerator temperature of 720.degree. C.
iv) Higher regenerator temperature requires better lining and metallurgy of the regenerator which increases the capital expenditure.
Therefore, it is essential to keep the regenerator temperature within limits below 700.degree. C. and preferably within 680-690.degree. C. to minimize the above damaging effects but at the same time without reducing the coke burning rate to less than an acceptable level. Unfortunately, most of the present resid FCC regenerators operate at high temperatures in complete combustion mode. Regenerator temperature to some extent can be reduced by partial combustion of the coke and by installing a CO boiler to take care of the unconverted carbon monoxide. However, with partial combustion regenerators, particularly at very high coke on catalyst as in resid FCC, it is difficult to maintain uniformity of bed temperature and after burning. Also, the catalyst inventory required to maintain sufficient residence time of the catalyst in the regenerator goes up with a reduced coke burning rate at lower temperatures. Therefore, running a partial combustion regenerator with resid spent catalyst is not commonly observed now a days.
Another way to control the regenerator temperature in resid FCC, is use of an external catalyst cooler with a suitable cooling capacity; regenerator temperature may be reduced by 20-30.degree. C. However, catalyst cooler is not desirable normally from the heat efficiency point of view since steam is generated in the cooler essentially at the cost of feedstock.
An important point to note is that the present resid FC units, even with a catalyst cooler, can handle feed Conradson coke up to 6-8 wt. % maximum residues heavier than this level, which can not be processed within the scope of existing resid FCC technology. Therefore, controlling the regenerator temperature is one of the key issues in resid FCC operation.
High Metal Level and Catalyst Deactivation
Residues contain large amounts of undesirable metals, e.g., vanadium, nickel, sodium, iron, copper, etc., which are poisonous to the catalyst activity and selectively. The poisoning effect of these metals aggravates exponentially at higher regenerator temperature. Recent residue crackers operating at high regenerator temperature (&gt;720.degree. C.) can tolerate maximum of 5000-8000 ppm of nickel and vanadium on the equilibrium catalyst. This ultimately raises the catalyst addition rate for bad quality of residue with higher metal content. Although nickel poisoning may be eliminated by using suitable nickel passivators effectively, the same is not true for vanadium.
Another important issue on metal poisoning is that most of the known resid crackers operate with close to zero carbon level on the regenerated catalyst. This has been found to be actually favorable for vanadium poisoning reaction. It has been proved earlier that (Ref ACS Symposium on Reduce Crude Cracking Catalyst by Ashland) maintaining a small but finite amount of coke on the regenerated catalyst is important to minimize oxidation beyond V.sub.2 O.sub.4 to V.sub.2 O.sub.5 with ultimately poisoning of zeolite active sites. Therefore, one way to improve the vandium tolerance of the catalyst is to not bottom the coke on catalyst to zero level.
The third important issue on metal poisoning originates from the inefficient stripping of the catalyst. Present FCC strippers operating in bubble bed are not very efficient at removing entrapped hydrocarbons from the catalyst. The problem becomes more acute with residue resulting in more carry over of unstripped hydrocarbons to the regenerator. Also, a lot of steam injected at the bottom of the stripper actually bypasses to the regenerator. Such steam present in the regenerator deactivates the acid sites of the catalyst much faster particularly at high regenerator temperature.
Crackability of Aromatic Resids
Residues, particularly aromatics, are not easily crackable in the present riser condition. The average temperature of the present risers are in the range of 540-560.degree. C. which is not enough to vaporize the residues completely. Moreover, the residence time in the riser is also quite low (2-6 secs) which may not be sufficient to crack the stable aromatic compounds. Also, it is known that a lot of the active sites are instantaneously deactivated at the riser bottom due to what is called "con.coke poisoning" of the zeolite pore mouth. It is thus believed that the present riser conditions are not sufficient to crack aromatic heavy fractions substantially. High temperature and residence time are to be kept to allow such aromatics to be cracked in the risers, as well as near complete vaporization of the residues to reduce coke make.
Poor Stripping Efficiency with Resid
The known resid FCC strippers are not considered efficient. It is observed commercially that the optimum stripping steam requirement for resid operation is 3-4 times more than that for corresponding clean feedstocks. This is because the heavier components of the resids are not easily strippable from the catalyst pores due to their relatively lower diffusion rates. Also, at the present stripper temperature of 510-530.degree. C. a lot of unvaporized hydrocarbons remain in the liquid phase inside the catalyst pores which are very difficult to strip in bubble bed mild stripping conditions.
One way to increase the stripping efficiency, particularly for resid operation, was proposed by Krambeck et al. in U.S. Pat. No. 4,481,103 where the stripper temperature was enhanced by circulation of regenerated catalyst partially in the stripper. The higher stripper temperature helps to remove the entrapped hydrocarbons more efficiently.
Still another way to improve the catalyst stripping was suggested very recently by H. Owen in U.S. Pat. No. 5,284,575. The concept of fast fluidized bed stripping is outlined in this Patent which addresses bad contacting of the bubbling bed stripping and proposes a high velocity efficient catalyst stripping. Nevertheless, such fast fluidized stripping also may not be fully adequate unless that stripping temperature is kept particularly for resid hydrocarbon where the unstripped material also contains components in liquid phase. Therefore, increasing stripper temperature and steam velocity are the two key issues to improve overall stripping efficiency for residue crackers.
Higher SO.sub.x and NO.sub.x Generation SO.sub.x and NO.sub.x generation in residue crackers are much higher than in conventional FCC units. This is partly due to higher regenerator temperature in resid cracking as outline previously. SO.sub.x additives which are used for reduction of SO.sub.x in flue gas, also looses efficiency at high regenerator temperature. Similarly, CO promoter additives, used in high temperature regenerators for resid, also retards the effectiveness of SO.sub.x additives. Poor stripping efficiency does not allow proper hydrolysis of the SO.sub.x additive in the stripper and thus badly affect the performance of this additive.
NO.sub.x generation is substantially increased with higher regenerator temperature (&gt;710.degree. C.). The situation become more critical if hot spots are generated in the regenerator bed due to inefficient axial and radial mixing. Increasing excess oxygen level may also contribute to the generation of NO.sub.x level. Therefore, from the SO.sub.x and NO.sub.x generation point of view, todays high temperature dense bed regenerators are not al all efficient. Similarly, the efficiency of SO.sub.x additives are also not found sufficiently good in present regenerator conditions. Therefore, to improve the SO.sub.x and NO.sub.x emission, resid FCC regenerators must address the high temperature and non-uniformity in bed profiles of existing reside regenerators.
Therefore, much of the problems faced by present resid cracking units originate in excess heat generation due to extra coke and inefficient heat balance between the rector and regenerator. Although, catalyst coolers are beneficial to reduce the regenerator temperature to some extent, the excess heat required to be removed, particularly for very heavy residues, demand an extremely large cooling capacity means which are to mechanically installed outside the regenerator. Moreover, use of catalyst coolers makes the overall system less energy efficient since the steam generated in the cooler is at the cost of the feedstock itself.
Another approach followed recently is descried in U.S. Pat. No. 4,336,160 by Dean et al. The first stage of the regenerator operates at lower temperature (650-675.degree. C.) and the second stage operates at much higher regenerator temperature (720-740.degree. C.). Although, it is claimed that the staging of the regenerator helps for reducing hydrothermal deactivation of the catalyst, ultimately the catalyst addition rate is not reduced effectively. This is because the overall catalyst inventory in the two stage regenerator is relatively higher than the single stage regenerator. Therefore, to reduce the catalyst addition rate it is not only important to reduce the catalyst deactivation but also the overall catalyst inventory should be as less as possible. Another, major draw back in the two stage regeneration, is that the flue gas of the first stage contains sufficient CO which needs to be burnt to CO.sub.2 in a separate incinerator, thus adding to the overall capital expenditure.
The most important of all, is the fact the known resid cracker are able to handle only mild residues with limited CCR (up to 5-8%) and metals. It is absolutely necessary to improve the heat and metal management of the system for processing heavier residues in a profitable manner.
The difficulty in resid FCC is that the riser/reactor and stripper temperatures should be maximized where as the regenerator temperature is to be minimized. This does not happen at all in conventionally heat balanced operations since any increase in the riser temperature essentially leads to an increase in the regenerator temperature also. Therefore, any new configuration where the gap between the riser/stripper and the regenerator temperature is brought down from the present level of 200-230.degree. C. to a lower value is going to high desirable.